1. Field of the Invention
The present invention relates to a tensioning device, and more particularly to a thread tensioning device for a sewing machine.
2. Description of the Prior Art
A typical thread tensioning device for a sewing machine is shown in FIGS. 4 and 5 and comprises two plates 80, 81 engaged on a shaft 82, a spring 83 for biasing the plates 80, 81 toward each other, and a knob 84 threadedly engaged on the shaft 82 for adjusting the resilient force of the spring 83 against the plates 80, 81 in order to adjust the clamping force against the thread 90 which is clamped between the plates 80, 81. However, when the thread 90 is clamped between the plates 80, 81, the plates 80, 81 have a tendency to form an opening outer peripheral portion due to the thickness of the thread 90, such that the thread 90 has a tendency to move radially outward of the plates 80, 81 and to move away from the shaft 82 (FIG. 5) during sewing operations. The thread 90 thus may not be stably retained in place such that the sewing operation will not be stable.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the conventional thread tensioning devices.